As the art of color cathode ray tube manufacture advances, the well-known phosphor-dot screen type of structure is being replaced by the so-called "slotted mask" construction. As is well known, the metallic aperture mask of the phosphor-dot structure included a plurality of substantially round holes. Therein, the diameter of the hole was the critical dimension required to determine the optical transmission characteristics of the holes in the aperture mask. Thus, it was found that relatively simple aperture measuring apparatus employing a light source and light detector was adequate to provide the information necessary to relatively consistent and repetitive manufacture of the aperture masks.
However, the advent of the "slotted mask" structure presented measurement problems which were unknown in the phosphor-dot structure. More specifically, the "slotted mask" structure includes a viewing screen having a plurality of phosphor stripes as compared with phosphor dots. Also, the "slotted mask" structure includes a multiplicity of spaced slots with each slot having multiple dimensions, such as length and width, subject to variation. Since a great number of dimensional combinations provide the same optical transmission capabilities, it becomes necessary to determine the dimensions of the slots if a consistent and repetitive product is to manufactured.
One known technique for determining the dimensions of the slot of a "slotted mask" structure as disclosed in the previously-mentioned application, utilizes a comparator mask in conjunction with a densitometer. Herein, a comparator mask having alternate opaque and transparent sectors overlays a slotted mask, and is disposed intermediate a light source and a light detector. The comparator mask and slotted mask are positionally located by a programmed computer and optical transmission readings are obtained which may be utilized to calculate the dimensions of the slots of the slotted material.
Although the above-mentioned techniques has been utilized with varying degrees of success, it is well known that the present-day type of "slotted mask" structure may include slots which not only vary in dimensioned size at different positional locations but also vary in spacing between slots at different positional locations. Also, it is common to have "slotted mask" structures wherein the slots tend to be slightly rotated from a vertical axis as the positional location advances from the center toward the outer edges of the structure.
More specifically, slotted masks for cathode ray tubes for example are usually of a rectangular configuration and the slots at the center of the mask are not usually of the same width or length as the slots in the corners of the rectangular-shaped mask. Also, the spacing between the slots at the center of the mask structure and the slots at the corners of the mask structure are known to be different. Moreover, it is not uncommon to have the slots form a series of arcs advancing outwardly from the center of the mask structure and these arcs are formed of slots which are at varying angles of rotation with respect to the horizontal and vertical axis.
As a result, it has been found that a comparator mask with a single section of alternate transparent and opaque sectors is less than adequate in determining measurements in most known "slotted mask" structures. Factually, the variations in slot spacing, slot width and slot rotation at different locations of the "slotted mask" would not permit proper alignment of the single section of alternate transparent and opaque sectors of the comparator mask therwith. As a result, a mismatch between the "slotted mask" and the comparator mask prevented a proper determination of slot size and a proper and consistent product of manufacture. Moreover, the above-mentioned slot, space and rotational variations in the "slotted mask" are deemed desirable which suggests the need for an improved comparator mask if manufacturing capabilities are to be maintained.